Comet 67P/Churyumov-Gerasimenko by Rosettas OSIRIS narrow-angle camera on 3 August from a distance of 285 km. The image resolution is 5.3 metres/pixel.

Summary

Periodic comet 67P/Churyumov-Gerasimenko has a nucleus measuring about 5 by 3 kilometers across that rotates once every 12.7 hours. It belongs to the Jupiter family of comets (comets with periods less than 20 years). The comet was discovered in 1969. Although it then had an orbital period of 6.55 years, an analysis of its orbit reveals the period had been longer in the recent past. During the early years of the 20th century, the orbital period had been about 9.3 years. A close approach to Jupiter in February 1959 (0.22 AU) reduced the period to 6.5 years. The comet has been seen at every return since its discovery.

Discovery

During mid-1969, several astronomers from Kiev visited the Alma-Ata Astrophysical Institute to conduct a survey of comets. On September 20, while still at Alma-Ata, Klim Ivanovic Churyumov examined a photograph exposed for periodic comet Comas Solá by Svetlana Ivanovna Gerasimenko on September 11.92, and found a cometary object near the edge of the plate which he assumed was the expected periodic comet. Upon returning to Kiev, the plates went under intense scrutiny. Precise positions were determined for all of the observed comets, as well as estimates of the coma diameter, and photographic magnitude estimates of the comet and nucleus. On October 22, it was realized that the position determined for P/Comas Solá was 1.8° from the expected position based on observations from other observatories. Further examination revealed P/Comas Solá in the proper position, near the limit of the photographic plate, which meant a new comet had been found. They estimated the magnitude of the new comet as 13, and said it had a faint coma 0.6 arc minute across, with a central condensation about 0.3 arc minute across. There was also a faint tail extending 1 arc minute toward PA 280 degrees.

In addition to the discovery observations noted above, additional images were found on a plate exposed by Gerasimenko on September 9.91 and on a plate exposed by Churyumov on September 21.93. The magnitude was estimated as 13 on the first date and 12 on the second.

This was the comet's first return after its discovery, but it was not a very favorable apparition. Astronomers at Palomar Observatory (California, USA) recovered the comet on 1975 August 8 and gave the nuclear magnitude as 19.5. They made further observations on September 9, October 6, and November 1, but provided no physical description. The final observation was made by astronomers at the Catalina Station of the Lunar and Planetary Laboratory (Arizona, USA) on 1975 December 7.

The comet was very favorably placed for observations during 1982. It was recovered by astronomers at Kitt Peak National Observatory (Arizona, USA) on 1982 May 31, when the magnitude was estimated as 19. The closest distance from the Sun came on November 12 (1.3062 AU) and the closest distance from the Earth came on November 27 (0.39 AU). Interestingly, the comet continued to brighten throughout December as it headed away from both the Sun and Earth, with amateur astronomers finding total magnitudes of 9 to 9.5. Around Christmas, Alan Hale (California, USA) was even able to detect the comet with 10x50 binoculars. The comet was last detected on 1983 May 13 by astronomers at Oak Ridge Observatory (Massachusetts, USA).

This was not a particularly good apparition for observations. The comet was recovered on 1988 July 6 by astronomers at Palomar Observatory (California, USA), at which time the nuclear magnitude was estimated as 20. It was only followed until 1988 September 12, when astronomers at Mauna Kea Observatory (Hawaii, USA) gave the nuclear magnitude as 18.6. The comet passed less than 4 degrees from the Sun in 1989 March and although it could have been observed near the end of 1989 and during the first half of 1990, no observations were made. After passing less than 3 degrees from the Sun in 1990 October, the comet again exited the Sun's glare and was observed by astronomers at Kitt Peak National Observatory (Arizona, USA) on 1991 May 15-16. They gave the nuclear magnitude as 21.8-22.0.

The 1996 appearance was another rather favorable one, although the comet never came closer than 0.9040 AU from Earth (1995 October 7). The comet had become brighter than magnitude 13 at the end of 1995 and continued to brighten. It passed perihelion on 1996 January 17, and with the distances from the Earth and sun increasing thereafter, it continued to brighten for another month. After reaching a maximum brightness of nearly 10.5 in February the comet faded and had dropped below magnitude 13 my mid-April. The coma diameter never exceeded two arc minutes during this apparition. the comet was last detected on 1995 May 31, when about magnitude 22.

The comet was recovered on 2002 June 18, when the magnitude was given as 15.0. It brightened to about magnitude 12.5 around the beginning of October. The comet was last detected on 2005 May 14, when about magnitude 22-23. The European Space Agency announced on 2003 May 28 that the Rosetta comet-chasing space probe will have a new target: periodic comet Churyumov-Gerasimenko.

This would prove to be another very favorable apparition for this comet. It was recovered by Gustavo Muler (Observatorio Nazaret, Teguise, Spain) on 2008 June 1 when around magnitude 19 and brightened to a maximum magnitude of about 9.5 in the days surrounding 2009 April 1. It entered the Sun's glare at the beginning of July and was not seen again until 2010 March when around magnitude 19. The comet was last detected from Observatoire Chante-Perdrix (Dauban, France) on 2010 July 7, when the magnitude was given as 19.9-20.3.

Rosetta

After a journey of about 10 years, Rosetta went into orbit around the nucleus of comet 67P on 2014 August 6. As the space probe approached the comet, several photographs and scientific measurements were made. Between July 13 and 21, the VIRTIS instrument was used to measure the comet's surface temperature. The result was about -70 degrees C. If the comet was completely covered in ice, it would have been 20-30 degrees C colder than this, so this indicates the surface of the comet is a "dark, dusty crust." On July 14, a series of images were taken which were used to create a movie of the rotating nucleus of the comet.